Works:
SET lc_time_names='en_US';
Fails with "Variable 'lc_time_names' can't be set to the value of 'de_DE'":
SET lc_time_names='de_DE';
This seemed simple to me. I guess, I have to install that locale but no search for "mariadb install locale" or "mariadb add locale" returned anything helpful. So I thought the available locales might be tied to the OS. I am using macOS, so locale -a lists me locales galore, "de_DE" is among them.
I ran the exact same database with that locale without problems but had to update macOS and reinstall php via brew install php@8.0. This updated more than just php. Afterwards, the error appeared.